A continent is a giant insulating blanket that does not allow mantle heat to escape very effectively. This image is of shear wave velocity beneath New Mexico where hot material is trapped beneath the North American plate. The hot material is causing rifting to begin at the Rio Grande Rift.

Supercontinent Breakup

As heat builds up beneath a supercontinent, continental rifting begins. Basaltic lavas fill in the rift and eventually lead to seafloor spreading and the formation of a new ocean basin. This basalt province is where Africa is splitting apart and generating basalt lava.

In the Afar Region of Ethiopia, Africa is splitting apart. Three plates are pulling away from a central point.

The Breakup of Pangaea

At the end of the Paleozoic there was one continent and one ocean. When Pangaea began to break apart about 180 million years ago, the Panthalassa Ocean separated into the individual but interconnected oceans that we see today on Earth.

The Atlantic Ocean basin formed as Pangaea split apart. The seafloor spreading that pushed Africa and South America apart is continuing to enlarge the Atlantic Ocean (
Figure
above
).

As the continents moved apart there was an intense period of plate tectonic activity. Seafloor spreading was so vigorous that the mid-ocean ridge buoyed upwards and displaced so much water that there was a marine transgression. Later in the Mesozoic those seas regressed and then transgressed again.

Growth of Continents

The moving continents collided with island arcs and microcontinents so that mountain ranges accreted onto the continents’ edges. The subduction of the oceanic Farallon plate beneath western North America during the late Jurassic and early Cretaceous produced igneous intrusions and other structures. The intrusions have since been uplifted so that they are exposed in the Sierra Nevada Mountains (
Figure
below
).

The snow-covered Sierra Nevada is seen striking SE to NW across the eastern third of the image. The mountain range is a line of uplifted batholiths from Mesozoic subduction.

Summary

Continents keep mantle heat from escaping, which may eventually lead to continental rifting.